This invention relates generally to DC-DC power converters.
There are two popular approaches to the design of DC-DC converters, both of them convert a DC voltage source into an AC source switching power semiconductor switches on and off at a high frequency. The AC source is then rectified to DC output. These approaches provide the means for achieving small, light-weight, and highly-efficient converters.
One approach of the DC to AC process is to generate a stream of pulses and to control the output voltage by controlling the width of the pulses. This pulse-width modulation (PWM) approach results in square-wave voltage waveforms across the switches and switching losses tend to be high and the electromagnetic interference (EMI) that accompanies the process is also high and difficult to control by filtering.
A second approach to DC-AC conversion is to add capacitor-inductor resonating elements to the PWM configurations in order to obtain sinusoidal voltage and/or current waveforms and achieve zero voltage or zero current switchings. These resonant converters have lower switching losses, thereby permitting operation at higher switching frequencies. The EMI generated by resonant converters is lower, and the higher switching frequencies result in reduction in size, weight and cost. However, resonant switching means that the semiconductor switches are subjected to greater stress, and switches designed for greater stress also have larger "on" resistance which tends to increase switching losses.
Both PWM converters and resonant converters do not have control on the amount of power transfer. In the case of short circuit of the output, the amount of power transfer increases drastically which result in destruction of the semiconductor switches or overheating.
There is a need for DC-DC converters that combines the simplicity of PWM converters with the low loss characteristics of resonant converters, yet it is desirable for the converters to have power limits for reliable operation.